1. Field of the Invention
The invention relates to non-invasive diagnosis of the internal state of batteries and more particularly to the use of the acoustic emission technique for diagnosis of the internal state of batteries.
2. Description of the Prior Art
Determining the state of charge (SoC) and the state of health (SoH) of a battery is an essential criterion for characterizing the internal state of the storage elements, in order to optimize the energy discharge/charge, as well as the life of the accumulators. Knowing these criteria is all the more critical for Li-ion batteries since an ill-controlled charge for this technology can lead to thermal runaway of the cell, or even to accumulator destruction. It is therefore necessary to develop a physical measuring technique allowing characterization of the internal state, identification of defects and prognosis during service.
There are many methods of estimating the state of charge (SoC). These methods can be grouped together into three main families:                from a physical measurement by detecting a physical change in the battery following charge/discharge, for example by measuring the density of the electrolyte during discharge. This method is suitable only for stationary batteries for which the electrolyte is involved in the reaction (lead batteries for example);        from voltage, current and temperature measurements.        
A first method uses coulomb-counting when the input and output current are measured and integrated in order to know the state of charge variation from a known state. This method however leads to estimation errors by disregarding phenomena such as self-discharge. Using other indicators, such as the no-load voltage or the estimation of an internal resistance as for example disclosed in U.S. Pat. No. 6,191,590 and EP Patent No. 1,835,297 A1, is also known. In this type of method, the SoC is first associated with one or more measurable or readily estimable quantities (potential, internal resistance, temperature) through the agency of static maps or of analytical functional dependencies. This type of method can in particular be used for recalibration of a coulomb-counting method during shutdown phases. However, these dependencies are in reality much more complicated than what is normally taken into account in the BMS, which often leads to SoC estimation errors. Finally, more complex methods are based on an algorithm that calculates in real time the state of charge using as input variables the “voltage-current-temperature” triptych. This alternative approach is based on mathematical battery models in order to use estimation techniques known in other fields. Patent application US-2007/0,035,307 notably describes a method of estimating the state variables and the parameters of a battery from service data (voltage U, current I, temperature T), using a mathematical battery model. The mathematical model comprises a plurality of mathematical sub-models and allows faster response. The sub-models are models of equivalent electrical circuit type, referred to as RC models, associated with restricted frequency ranges. Another SoC estimation method known in the literature ([Gu, White, etc.]) is based on the mathematical description of the reactions of an electrochemical system. The SoC is calculated from state variables of the system. This description rests on material balances, charge, energy, and on semi-empirical correlations. All these methods can be used in stationary or on-board applications, as in vehicles;
from an electric load put on the battery and in particular the electrochemical impedance spectroscopy method that uses a low-amplitude frequency load: these methods can also be used for estimating the SoH of the batteries and they are described below.
Concerning the SoH estimation methods known in the literature, in WO-2009/036,444, a reference electrode is introduced in commercial elements in order to observe the degradation reactions of the electrodes. This method however requires substantial instrumentation, notably for inserting a reference electrode inside the element, and more complex electronic management of the battery.
French Patent 2,874,701 describes a method using a temporal electric perturbation in order to compare the response obtained with a reference response. However, this method is more difficult to implement for Li-ion type elements whose response variations following this type of perturbation are very low and can therefore not allow precise SoH measurement.
SoC or SoH estimations from electrochemical impedance measurements are numerous. The simplest one uses pre-recorded charts with different states of charge and different temperatures, so as to find the state of charge from an impedance measurement, knowing the temperature. This methodology is widely used in the laboratory for determining the internal state since there are indeed changes in the impedance depending on the SoC or the SoH.
A potentially more promising method is based on the measurement, by impedance spectroscopy (EIS), of a quantity parametrized by the SoC. For example, US Published Application 2007/0,090,843 determines by EIS the frequency f± associated with the capacitive-inductive transition. A correlation between frequency f± and the SoC is presented for a lead battery, as well as for Ni—Cd and Ni-MH batteries. A similar approach is based on the modelling of the EIS spectra by equivalent electrical circuits, whose components are parametrized by the SoC, as described in U.S. Pat. No. 6,778,913, which allows development of an automotive battery tester Spectro CA-12 (Cadex Electronics Inc., Canada) based on multi-frequency electrochemical impedance spectroscopy for the acid-lead pair. The EIS spectra are approximated by equivalent electrical circuits and the evolution of the components is parametrized by the SoC. Similarly, in U.S. Pat. No. 6,037,777, the state of charge and other battery properties are determined by measuring the real and imaginary parts of the complex impedance/admittance for lead batteries or other systems. The description of the electrochemical and physical phenomena at the electrodes and in the electrolyte serving as a support for the development of the RC model, the temperature of the battery being simulated by the model, in order to increase in precision, in relation to an external measurement is described in EP 880,710.
Impedance analyses have also been described in the literature. U. Tröltzsch et al. Electrochimica Acta 51, 2006, 1664-1672. describe a method wherein they use impedance spectroscopy coupled with the adjustment of impedances according to an electrical model so as to obtain the state of health of the element. This technique however requires stopping using the element for the measurement.
French patent application 2,956,486 filed by the applicant describes a method for diagnosis of the internal state of a battery such as the SoC or the SoH using impedance measurements modelled by means of an equivalent system. A multivariate statistical analysis allows calibration of a relation between the SoC (and/or the SoH) and the parameters of the equivalent circuit characteristic of a given battery.
However, conventional diagnosis techniques using electrical measurements, such as impedance spectroscopy, remain complex techniques which are difficult to implement. Estimation of the internal state of a battery is improved if a complete diagnosis is available including state of charge and state of health, as well as the failure state of one or more elements of the battery. A non-invasive technique allowing fast diagnosis of the internal state, involving anticipation of a possible failure, of the battery is therefore required.
The acoustic emission technique allows detection of the failure of a battery even when failure cannot be detected by electric measurement. The principle of using acoustic emission to study the physical state of batteries or of components (electrodes for example) has been the subject of many academic surveys concerning notably the study of ion insertion mechanisms during charge/discharge, highlighting the sensitivity of the technique to ion insertion/disinsertion phenomena in the Li-ion battery electrode materials (for example Kalnaus, S., K. Rhodes and C. Daniel, “A study of lithium ion intercalation induced fracture of silicon particles used as anode material in Li-ion battery”, Journal of Power Sources in Press, Corrected Proof (2011)), and to the phenomena of electrochemical and mechanical decrepitation of electrodes (for example Etiemble, A., H. Idrissi and L. Roue “On the decrepitation mechanism of MgNi and LaNi5-based electrodes studied by in situ acoustic emission”, Journal of Power Sources 196.11 (2011): 5168-73).
However, these studies do not aim to correlate the internal state of a battery with the acoustic emission from battery elements during charge/discharge cycles.
Japanese Patent 7,006,795 describes a device allowing detection of an acoustic emission signal from a battery and to separate this signal into two signals according to their frequency so as to identify the generation of gas or the deterioration of the internal structure of the battery.
Patent application WO-11,001,471A describes a device for detecting the internal state of a battery, which can detect by acoustic emission detectors short elastic waves accompanying a reaction in a battery and which can check with precision changes in the internal state of a battery.
French patent application 2,949,908 describes a method of monitoring an electrochemical accumulator on charge or in service, which comprises a stage of direct detection of an anomaly in the accumulator, such as the progress of at least one harmful chemical reaction within the accumulator or a physical degradation of the accumulator.
However, these documents do not describe a complete method for precise diagnosis of a given internal state of a battery, including the defectiveness and/or the state of charge and of health of the battery.
Surprisingly, tests have shown that the acoustic emission technique can allow not only monitoring of the evolution of the internal state of various batteries by recording the acoustic signals produced when the batteries are operating, but also can highlight an acoustic signature of a fault or of a given internal state of the battery, through signal processing performed on the signal records. A series of comparative tests on one or more reference electrochemical systems allows defining of a relation between a given internal state and an acoustic signature which later enables fast and non-invasive diagnosis on a given electrochemical system.
The method and the system according to the invention allow determination of the internal state of a battery, notably its SoH, its state of charge SoC or its failure state, using the acoustic emission technique.